3.3 Ga magmatic events in the Anshan area:new SHRIMP age and geochemical constraints

Abstract

Anshan is one of the very few localities in the world where abundant Archaean geological records are preserved. The SHRIMP age data and geochemical results of the Palaeoarchaean dioritic gneiss and fine-grained trondhjemite of the ancient Dongshan rock belt from the Anshan area are reported in this paper. The dioritic gneiss is rich in rare earth elements (ΣREE = 145×10 -6 ), and the light REE is weekly differentiated from the heavy REE [(La/Yb)n = 2.8]. The fine-grained trondhjemite is slightly rich in REE (ΣREE =169×10 -6 ), and the light REE is strongly differentiated from the heavy REE [(La/Yb)n = 49.9]. Weighted average 207 Pb/ 206 Pb ages of 3321 ± 6 Ma and 3321 ± 5 Ma have been obtained for the dioritic gneiss and trondjhemite respectively. It is demonstrated that the late Palaeoarchaean is a significant stage of the continental crust growth in the Anshan area, witnessing two types of magmatism, one originated from the depleted mantle source and an-other from recycled crustal source.